This invention relates to the deposition of conformal aluminum films onto a refractory metal nitride layer and to the formation of void-free aluminum plugs in via and contact openings in semiconductor devices.
In the fabrication of integrated circuits, electrical contact must be made to isolated active device regions formed within a semiconductor wafer/substrate. The active device regions are connected by highly electrically conductive paths or lines which are fabricated above an insulator material which covers the substrate surface. To provide electrical connections between the conductive path and active device regions, an opening in the layer of insulator material is provided to enable the electrically conductive films to contact the desired regions. Such openings are typically referred to as contact openings or simply contacts. Ultimately, an electrically-conductive filling material such as tungsten metal would be provided in the contact opening for making electrical connections between the surface conductive paths and the active device regions on the substrate.
As transistor active area dimensions have approached less than one-half micron in diameter on ultralarge scale integrated circuit (ULSI) devices, conventional process techniques have proved to be unable to meet the needs of providing low resistance paths to the contacts. One of the more difficult problems in semiconductor device fabrication has been the filling of via or contact openings that have only submicron diameters and high aspect ratios (i.e., high depth to diameter ratios) of 4:1 or greater with metal. Sputtering techniques do not provide good step coverage at such high aspect ratio openings.
Chemical vapor deposition techniques do provide better results in filling such openings. Tungsten metal has been used as the plug material to fill high aspect ratio via and contact openings in semiconductor device manufacture. However, tungsten, and the processes used to deposit the tungsten metal, have several drawbacks. Current chemical vapor deposition techniques for tungsten (W) use a tungsten fluoride gas (such as WF.sub.6). The fluorine atom in the gas will attack the silicon surface of a semiconductor substrate. The tungsten metal itself may react with the silicon substrate. Additionally, while tungsten has reasonably low electrical resistivity, there are other metals with even lower resistivities which could provide a better electrical contact and flow path.
To prevent unwanted contact with and possible reaction with underlying layers, electrically conductive barrier layers have been interposed. Refractory metal nitrides such as titanium nitride have been used because of their ability to prevent interlayer diffusion. Titanium nitride has been preferred as it is electrically conductive, chemically stable, and has good barrier properties.
Chemical vapor deposition of aluminum has become increasingly important in the manufacture of semiconductor devices. Aluminum has always been used as a conductor material because of its properties of high conductivity, an electrical resistivity less than that of tungsten, high adherence, and low stress. Aluminum is thus a desirable candidate to serve as the electrically conductive material to fill the contact openings. However, aluminum films deposited using chemical vapor deposition (CVD) techniques have heretofore had rough surfaces, voids, and have not been conformal without further surface modification. Such surface roughness may lead to problems with later photolithography steps resulting in a decrease in resolution. Further, aluminum which has been deposited into vias or contacts using chemical vapor deposition techniques has exhibited voids and notches, especially at the interface between the aluminum and refractory metal nitride interface, resulting in poor electrical connections.
Attempts have been made to improve the quality of CVD aluminum films. It is believed that substrate pretreatment using titanium-containing gases improves the surface morphology of aluminum films by providing nucleation sites. One prior process uses chemical vapor deposition of TiCl.sub.4 gas to pretreat wafer substrates prior to aluminum deposition. The TiCl.sub.4 molecules adsorb onto the substrate surface. However, residual chlorine at the interface between the substrate and aluminum has been known to cause corrosion of the aluminum metal.
More recently, metalorganic titanium compounds have been used to deposit titanium directly onto silicon and silicon dioxide surfaces as a pretreatment for later CVD of aluminum. See, Sugai et al, "Aluminum Chemical Vapor Deposition with New Gas Phase Pretreatment Using Tetrakisdimethylamino-titanium for Ultralarge-scale Integrated-Circuit Metallization", J. Vac. Sci. Technol. B 13(5), Sept./Oct. 1995, pp. 2115-2118. However, again, problems may arise as the directly-deposited aluminum metal may attack the silicon and silicon dioxide surfaces.
Accordingly, the need still exists in this art for a technique and process of depositing a smooth and conformal layer of aluminum and to produce a void-free electrically conductive plug in via, contact, or trench openings for integrated circuits.